THE SHELL. 23 



curious ways in which this is sometimes obtained. The Neritidse, 

 Helicinidae and Auriculidfe (1, 14) dissolve all the internal spiral 

 column of their shells ; the cone (i, 8) removes all but a paper- 

 like portion of its inner whorls ; the cowry goes still further, and 

 continues removing the internal layers of its shell-wall, and 

 depositing new layers externall}' with its overlapping mantle 

 (Ixi, 99), until, in some cases, all resemblance to the young shell 

 is lost in the adult. 



The power which mollusks possess of dissolving poi^tions of 

 their own shells is also exhibited by the Murices in removing 

 those spines from their whorls which interfere with their growth ; 

 and by the Purpura and others in wearing away the inner wall 

 of their aperture. 



Decollated Shells. It frequently happens that as spiral shells 

 become adult they cease to occupy the upper part of their 

 cavity ; the space thus vacated is sometimes filled with solid 

 shell, as in Magilus ; or it is partitioned off, as in Vermetus, 

 Euomphalus,Turritella, Triton or Caecum (i, 11). The deserted 

 apex is sometimes very thin, and becoming dead and brittle, it 

 breaks awa}^, leaving the shell truncated or decollated. This 

 happens constantly with the Truncatell^, Cylindrellse (i, 12, 13), 

 and Bulirtius decollatus ; amongst the fresh-water shells it 

 depends upon local circumstances, but is very common with 

 Pirena and Cerithidea. 



Decollated shells usually have the whorls of the spire closely 

 wound and not increasing much in diameter. Before the trun- 

 cation takes place the partition or septum is formed which is 

 intended to close the new summit. A number of tropical and 

 subtropical terrestrial species, particularly of America, uniformly 

 truncate their shells, the portion closed off from contact with the 

 mollusk becoming dead and fragile so as to break away readil}'". 

 M. Gassies has. however, observed Rumina decollata strike with 

 the summit of its shell, with the evident purpose of fracturing 

 it. In Caecum also (i, 11), which commences growth as a spiral 

 shell, a septum is formed soon after the close spire has been 

 exchanged for a simply curved tube, and then the spire is 

 detached. As the shell progresses in its growth another septum 

 is formed and again it suffers truncation at its initial end, so 

 that the adu.lt Caecum is a simple curved tube, differing from the 

 Scaphopoda or Dentalium shells in the small end being closed 

 instead of open, as in the "tooth shells." In the latter the 

 posterior opening is for the extrusion of the eggs — which in 

 these only, among encephalous mollusca, are accommodated 

 with a separate aperture in the shell for their passage. 



Forms of Shells. These will be described particularly here- 

 after ; enough has been said to show that in the molluscan shell 

 (as in the vertebrate skeleton) indications are afforded of many 



